This class is a factory for the family of FusqlFS::Backend::Base subclasses. Its new() method initializes and returns some kind of real backend class instance (FusqlFS::Backend::PgSQL or FusqlFS::Backend::MySQL for now) depending on `engine' option passed to it. If engine is not given or not recognized, it falls back to `PgSQL' backend with a debug message.

You better look at one of underlying classes for detailed description.